Mafia II

This is extremely easy and will probably be the first achievement you get in this game. Simply be at the very short tutorial level where you are sent to fight in a war. the achievement will pop after the cut-scene after using the MG42.

Story progression, achievement not missable.

Chapter 1: The Old Country

Storming the Army Stronghold

You and your unit will be advancing on an Italian base. Use your rifle to snipe the dictator on the balcony and start your assault. Move up the stars on the right and you'll witness a number of rebels be destroyed by a heavy machine gun. When the shooting stops, run passed the door and enter the window on the left. Take cover behind the walls and toss a grenade to take out the machine gun and the nearby soldiers.

Let your health recover before moving on. There is a small weapons cache, so arm up with what ever you find. Kick open the door on the right and support the rest of your unit as you make your way up the stars. The Italian army has taken hostages, so you may want to aim carefully. However, even if you do rescue them, they will still be shot down by another wave of enemy soldiers.

You'll be forced on to a balcony that is taking heavy weapons fire. Quickly take cover at pop out to take on the enemy. Your army buddies will take care of most of the hard work. However, as you make your way inside, an enemy soldier will kill one of your allies and point his gun straight at you. Quickly respond and kill off the threat ahead.

Move into the room to the left and arm yourself with the machinegun. It has infinite ammo, so there is no need to be conservative. Kill off as many enemies as possible and soon a tank arrives. Once the heavy artillery fires off its shell the chapter will conclude.

Source: http://www.gamespot.com/features/6274410/p-2.html